1. Field of the Invention
The various embodiments relate generally to polymer binders for porous composites used in energy storage devices, such as electrodes in primary and secondary batteries, double-layer capacitors, electrochemical capacitors, supercapacitors, electrochemical capacitor-battery hybrid devices, as well as dense (non-porous) composite dielectric layers in dielectric capacitors, and polymer separators for use in primary and secondary batteries, electrochemical capacitors, supercapacitors, double-layer capacitors, and electrochemical capacitor-battery hybrid devices.
2. Description of the Relevant Art
Growing efficient materials, components, and structures from plants are of the highest interest for the sustainable future, due to the preservation of the environment during the plant growing processes and a plant's ability to efficiently capture carbon dioxide. Particularly attractive are marine plants, such as algae, that can be grown on non-agricultural land, such as salt water or waste water, and need only a fraction of the area required by conventional crops.
Due to rapidly increasing renewable energy demands, energy harvesting by ocean plants has drawn interest in recent years. Equally important is the development of high-performance, eco-efficient components for energy storage devices, such as batteries. Several breakthroughs have recently been achieved in the formation of organic cathodes and anodes for lithium-ion batteries. These bio-derived active materials show great promise, however they offer limited stability and capacity properties.
A typical procedure for the preparation of Li-ion battery electrodes includes mixing electro-active powder with conductive carbon additives and a polymeric binder dissolved in a solvent. The produced slurry is then casted on metal foil current collectors and dried. Traditionally, most research has been focused on synthesis of active powders with improved properties and less attention was devoted to the advancement of the electrically inactive components of battery electrodes, such as binders. Yet, recent studies have shown that many important battery characteristics, including stability and irreversible capacity losses, are critically dependent on the binder's properties. High capacity electrochemically active particles that exhibit significant volume changes during insertion and extraction of Li require improved binder characteristics to ensure electrode integrity during use. Si, in particular, exhibits the largest volume changes during Li-ion battery operation. The interest in Si-based anodes stems from the abundance of Si in nature, its low cost, and its high theoretical capacity, which is an order of magnitude higher than that of the conventionally used graphite.
Recent studies have shown that synthetic and bio-derived polymers which contain carboxy groups, such as polyacrylic acid (PAA) and carboxymethyl cellulose (CMC), demonstrate promising characteristics as binders for Si-based anodes. Low binder extensibility did not demonstrate a negative effect on the battery performance. Reasonably stable anode performance, however, could only be achieved when Si volume changes were minimized by incomplete Li insertion in the tests or accommodated by using extra-large binder content, which lowers the resulting anode capacity. The polar hydrogen bonds between the carboxy groups of the binder and the SiO2 on the Si surface were proposed to exhibit a self-healing effect and reform if locally broken. An alternative explanation for the observed stability of the rigid binders with lower extensibility could be that Si nanoparticles deform plastically during electrochemical alloying with Li, expanding towards the existing pores between the particles.